Carbohydrates
We can divide carbohydrates into three main types: sugars, starches, and fiber. Of the three, we use sugars and starches as fuel.
Fiber is not used nutritionally unless it is first fermented by bacteria in the gut.
Since the mid-70s, carbs have been classified as simple or complex. An easy way to think about this is: simple carbs are sugars, and complex carbs are starches. It was generally recognized that we digest simple carbs more quickly than complex carbs.
These days, the classification of carbohydrates into either simple carbs or complex carbs is falling out of favor.
Now, nutritionists prefer to rank food sources of carbohydrates according to their place in the Glycemic Index.
What is the Glycemic Index?
The Glycemic Index (GI) is a measure of how quickly the blood sugar rises after consumption of a food source of carbohydrates.
Foods that are easily digested will cause the blood glucose level to peak quickly. These foods are ranked high on the Glycemic Index. Low-GI foods, on the other hand, take longer to digest and don’t cause a marked peak in blood glucose levels.
The US Government ranks foods based on their position in the GI. You can download this data in an Excel spreadsheet that lists the GI position of thousands of different foods here.
Why use the Glycemic Index?
The old system – classifying carbs as simple or complex – failed to take into account the fact that identical carbs are digested at different rates depending on the food in which they are found. The fructose from an orange, for instance is digested to glucose and released into the bloodstream at a much faster rate than the fructose found in a grapefruit.
The Glycemic Index is based on the food you eat, making it a useful tool when designing a diet.
How does the body use carbs?
Dietary carbohydrates, excluding fiber, are digested and enter the bloodstream as glucose, a simple sugar. Cells use glucose as fuel.
Insulin is a hormone that causes cells to absorb glucose from the blood. A high insulin level will “drive” glucose into cells, lowering the blood glucose concentration.
Insulin production is regulated by the blood glucose concentration: as blood glucose concentration rises, insulin production increases, and vice versa.
Ingestion of high-GI carbs results in a rapid spike in blood glucose concentration. This, in turn, causes insulin levels to rise dramatically as part of a compensatory regulatory mechanism called the insulin response. Glucose is forced into the cells at a high rate. Low-GI carbs do not stimulate as severe an insulin response.
The insulin response caused by high-GI carbs often overcompensates. That is, it causes blood sugar levels to drop below normal. This low blood sugar state causes light-headedness, fatigue, and hunger pangs.
By way of illustration, those of you who are semi-addicted to soda may be caught in a cycle of high-GI carb ingestion, insulin response, and the resulting low blood sugar which causes you to crave another soda.
How does the Glycemic Index relate to weight lifting?
As mentioned in the last section, cells use glucose for fuel. If insulin peaks soon after ingestion of high-GI carbs, blood sugar is forced into the cells that need it.
The body prefers to build fat cells. After the insulin response, these cells take sugar from the blood and convert it into fat stores.
However, when the muscle cells are in an anabolic state – i.e. actively engaged in protein synthesis – they need more than a usual amount of fuel. If blood sugar and insulin levels peak during this time, the body will shunt energy into protein synthesis and hypertrophy rather than fat storage. Obviously, this is a good thing for anyone involved in weight training.
When should I eat low-GI and high-GI carbs?
It is best to eat high-GI carbs immediately before and after lifting. At other times, low-GI carbs are preferable, since they don’t stimulate the insulin response to the same degree as their high-GI counterparts.
Eating high-GI carbs will preferentially drive sugar into fat cells except when your muscles are in an anabolic state. So if possible, try to eat low-GI carbs except right around the time when you are lifting weights with a high level of intensity.
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{ 5 comments… read them below or add one }
In reviewing the GI values for bread, it seems there’s not much variation between white bread and wheat bread. Is this accurate? Or am I not looking at the data properly? It seems like wheat bread should be much lower.
I wonder if the data in that list is completely accurate? Perhaps some other lists of GI values will differ. I think this relatively new focus on the Glycemic Index can sometimes confuse the issue(s) rather than help. Bottom line is that unprocessed foods are usually better for us.
I can’t wait to try this!! I am still alittle confused on whether this will help me to burn fat. I am flabby and have taken on weight lifting for exercise because I hear that it kicks up the metabolism stronger and for a longer period than cardio alone does.
Hey Nolan if you want to burn fat here is a little trick. Whichever area you want to burn the fat in do your weight lifting first then do your cardio….do 10 minute intervals (walk a minute then run a minute). The weight lifting will burn your sugars first and then when you do cardio it will burn fat and target that area more since it is trying to recover. I do this with my abs and I have seen results. Hope this helps bud.
Some very interesting info here. Just wondering about something–my understanding is as follows: If we eat a high GI food such as white rice, potatoes, etc., the carbs are digested and released as glucose into the blood very quickly causing a blood sugar level spike. Insulin is then released to transport the glucose into cells.
Question 1: Does a higher blood sugar level after cause a greater % of the glucose to be stored in fat cells as fat i.e. because there is more glucose available than is needed by the body for immediate use after blood sugar is elevated to a certain level?
Further–low GI foods digest more slowly. Question 2: Are researchers sure that with low GI foods the end result is that the release of carbs as glucose into the bloodstream happens not only after a longer time period has elapsed, but also more gradually? Are they sure that the end result is that blood glucose never reaches the levels seen as when high GI foods are eaten?